Ikiryou
Mythology & Folklore

for further reading: Wikipedia

In Japanese mythology, an ikiryou (also known as shoryo, seirei, or ikisudama) is a manifestation of the soul of a living person separately from their body.

Traditionally, if someone holds a sufficient grudge against another person, it is believed that a part or the whole of their soul can temporarily leave their body and appear before the target of their hate in order to curse or otherwise harm them, similar to an evil eye.

Souls are also believed to leave a living body when the body is extremely sick or comatose; such ikiryou are not malevolent.

Relevance to Kyokai no Rinne

Reiji Todoriki becomes an ikiryo after his motorcycle accident.